There has conventionally been known multi-beam printers, which scan a photosensitive drum with a plurality of laser beams, perform an image forming process, and thereby achieve printing by laser printer at both high resolution and high speed.
<Configuration Using Single Pixel Clock Generation Unit for All Laser Beams>
A schematic configuration using a single pixel clock generation unit for all laser beams will be explained with reference to FIG. 2. FIG. 2 is a block diagram showing a schematic configuration using a single pixel clock for all registers, which generate a video signal in a multi-beam printer for forming an image with four laser beams.
In FIG. 2, an image processing unit 263 receives print data, internally executes image processes such as a smoothing process and toner saving process, and then outputs the resulting data to registers 264a to 264d. The registers 264a to 264d output print data as video signals to a printing engine 28 in synchronization with a clock output from a pixel clock generation unit 265. The pixel clock generation unit 265 outputs a pixel clock whose phase is synchronized with a horizontal synchronizing signal output from the printing engine 28. The printing engine 28 comprises a plurality of (in this case, four) laser output control units which control laser outputs on the basis of four input video signals.
In the configuration of FIG. 2, all the lasers operate in accordance with pixel clocks of the same frequency, and the multi-beam printer suffices to have, e.g., a minimum number of PLLs (Phase Locked Loops) necessary to generate a pixel clock. Thus, the circuit of an image output unit can be downsized, and the multi-beam printer can be manufactured at low cost.
In the multi-beam printer, however, the optical path lengths are different between laser beams, and the scan widths of the laser beams in the main scan direction are inconsistent in the configuration using a single pixel clock generation unit for all laser beams, as shown in FIG. 2. The inconsistency of the scan widths causes a jaggy (jitter) of an image formed with the laser beams. The image quality degrades if the number of laser beams is large and the optical path difference is large. FIG. 7 is a schematic view showing the situation where the scan widths of the laser beams in the main scan direction are inconsistent, due to the difference of the optical path lengths between multi-laser beams in the prior art. 701 is a photosensitive drum, and 702 to 705 are laser beams respectively. In this example, there are 4 laser beams. As shown if FIG. 7, the difference of the optical path lengths on the surface of the photosensitive drum, results in the inconsistency of the scan widths in the main scan direction between laser beams, which causes a jaggy (jitter) of an output image.
<Configuration Using Different Pixel Clock Generation Units for Respective Laser Beams>
A configuration as shown in FIG. 3 has conventionally been known to solve the problems of the configuration which uses a single pixel clock generation unit for all laser beams, as shown in FIG. 2. FIG. 3 is a block diagram showing a schematic configuration in which individual pixel clock generation units are arranged for each laser beam in a multi-beam printer for forming an image with four laser beams.
In FIG. 3, an image processing unit 363, registers 364a to 364d, and printing engine 38 are the same as the image processing unit 263, registers 264a to 264d, and printing engine 28 in FIG. 2, and a description thereof will be omitted. The operation of pixel clock generation units 365a to 365d is the same as that of the pixel clock generation unit 265 in FIG. 2, and a description thereof will be omitted.
The configuration in FIG. 3 is different from that in FIG. 2 in that the pixel clock generation units 365a to 365d are arranged for the corresponding registers 364a to 364d to perform synchronous control. When individual pixel clock generation units are used for all registers, as shown in FIG. 3, the difference in scan width in the main scan direction between laser beams is corrected by individual pixel clocks even if the number of laser beams is large and the optical path difference between laser beams is large. Accordingly, a high-quality image can be formed.
Prior arts with these configurations have conventionally been known. Japanese Patent Laid-Open No. 2001-264654 discloses a configuration which multiplies a pixel clock to generate pixel clocks for respective laser beams. In this configuration, the scaling factor in the main scan direction is held as data, and the frequency is adjusted by referring to the data.
Japanese Patent Laid-Open No. 5-344292 discloses a configuration which divides a clock higher in speed than a pixel clock by a counter to generate a pixel clock.
Japanese Patent Laid-Open No. 2005-169870 describes that in the multi-beam printer, the scan widths of the laser beams in the main scan direction are inconsistent, due to the difference of the optical path lengths between laser beams. It also discloses a configuration which generates data for adjustment of the scanning start position by laser beams for each laser beam.
Japanese Patent Laid Open No. 5-294005 describes an image forming apparatus which has a laser array where the emission of luminescence part is arranged in the shape of two dimensions. It also discloses that a spot position of the laser is the same position in the main scanning direction, uses the same delay circuit in the image forming apparatus.
In a configuration as shown in FIG. 3, pixel clock generation units must be prepared for respective registers, and circuits such as PLLs must be arranged for all the pixel clock generation units. Thus, the circuit becomes complicated and large, and the whole system becomes expensive.